This diverse taxon contains free-living, commensal and parasitic species. One subphylum, the Kinetoplastida, harbors some of the most serious protozoan parasites of humans and other animals.
Life cycles of these parasites are diverse. They may be intracellular or extracellular, and usually have two hosts, transmitting via blood feeding vectors.
Its members possess flagella for locomotion and may possess an undulating membrane. There is just a single type of nucleus.